The present disclosure is related to thermoacoustic devices, and more specifically to an electrical control system for optimizing the operation of a thermoacoustic device such as a thermoacoustic refrigerator or thermoacoustic heat engine.
The pulse-tube refrigerator, an example of which is shown in FIG. 8, typifies travelling-wave thermoacoustic refrigerators. In device 10, an acoustic wave travels through a gas. The pressure and velocity oscillations of the gas are largely in-phase in certain regions of the device. Thus, these devices are generally referred to as traveling-wave devices. See, for example, U.S. patent application Ser. No. 12/533,839 and U.S. patent application Ser. No. 12/533,874, each of which being incorporated herein by reference.
In device 10, an acoustic source 12, for example an electromechanical transducer with a moving piston, generates oscillating acoustic energy in a sealed enclosure 14 containing compressed gas. Noble gases, such as helium, are often used, though many gases and combinations thereof, including air, can be utilized. The acoustic energy passes through a first heat exchanger, the “hot” heat exchanger 16, generally connected, for example via heat exchange fluid, to a heat reservoir at ambient temperature, a regenerative heat exchanger, or “regenerator” 18 (described below), and another heat exchanger, the “cold” heat exchanger 20, which is connected, for example via heat exchange fluid, to the thermal load which is to be cooled by the refrigerator. Usually, the cold heat exchanger is followed by another tube, called a “pulse tube,” 22 and a last ambient-temperature heat exchanger, the “ambient” heat exchanger 24, which serves to isolate the cold heat exchanger and thereby reduce parasitic heat loading of the refrigerator. The “hot” heat exchanger 16 and “ambient” heat exchanger 24 are often at the same temperature. After the “ambient” heat exchanger is an acoustic load 26, often an orifice in combination with inertances and compliances, which dissipates acoustic energy. Here, a “heat exchanger” is taken to mean a device which exchanges heat between a gas inside the thermoacoustic device and an outside fluid, such as a stream of air.
In steady state, a temperature gradient is established in the regenerator in the direction from the hot to the cold heat exchanger (if taken as a vector the gradient would be in the opposite direction). Heat is ideally transferred nearly isothermally between the gas and the regenerator material, often metal or ceramic porous material or mesh. With traveling-wave acoustic phasing, the gas in the regenerator undergoes an approximate Stirling cycle. In this way, the maximum heat can be moved from the cold to the hot heat exchanger per acoustic energy consumed.
FIG. 9 illustrates a looped travelling-wave thermoacoustic refrigerator device 30 of a type known in the art. In device 30, acoustic load (26 of FIG. 8) is replaced by an acoustic section 46 that delivers part of the acoustic energy that would otherwise be dissipated in the load to the back face of the electromechanical transducer 32, thereby reducing the electrical input power required for a given cooling power and therefore increasing the efficiency of the device. In another configuration disclosed in the aforementioned U.S. patent application titled “Thermoacoustic Apparatus With Series-Connected Stages”, Ser. No. 12/771,617, “excess” acoustic power is delivered to the back of an electromechanical transducer of a second thermoacoustic refrigerator, whose load is similarly replaced with an acoustic section that delivers its “excess” acoustic power to the back face of the first electromechanical driver in a closed loop. Similarly, three or more thermoacoustic refrigerator units can be connected, output-to-input, in a closed loop. In another device known in the art, the “excess” acoustic power is delivered to the front face of the electromechanical transducer.
Analogously, a traveling-wave thermoacoustic heat engine is a device which converts heat to work. FIG. 10 illustrates an embodiment 50 of such a device known in the art. In this device, heat is applied at “hot” heat exchanger 54, which is maintained at a high temperature. “Cold” heat exchanger 58 and “ambient” heat exchanger 62 are maintained at ambient or cold temperatures. Oscillating acoustic energy in the enclosure 52 is converted to electrical energy by a power transducer 66, for example, an electromagnetic transducer.
The temperatures in thermoacoustic coolers and heat engines are rarely fixed, but are functions of ambient conditions, heat availability, user settings, and so forth. When operated at a given power and frequency, the efficiencies of thermoacoustic refrigerators vary with the temperatures of the hot, cold, and ambient heat exchangers. Similarly, when operated at a given power and with a given load, the efficiencies of thermoacoustic heat engines vary with the temperatures of the heat exchangers. This effect is particularly significant in the case of a looped refrigerator (as in FIG. 9) or engine (as in FIG. 10) because such a system is resonant, with the resonant frequency depending in part on the operating temperatures, such as the temperatures of the ambient environment in which the device operates, the temperatures of the several heat exchangers, and so on, which affect the acoustic gain inside the regenerator, and, in the case of the engine, the load. As the temperatures change, the resonant frequency changes and hence the optimal frequency of operation changes. In the case of a pulse-tube refrigerator and like devices, as the temperatures change, the phasing of the acoustic power in the region of the regenerator changes, potentially reducing the effectiveness of heat regeneration and thereby the efficiency of the device. Therefore, there is needed in the art an apparatus and method for controlling aspects of the operation of a thermoacoustic device so as to optimize its efficiency as a function of the conditions of operation, such as temperature, humidity, etc.